CD4+ helper T-cell differentiate into at this time at least five well-defined subsets: Th0, Th1, Th2, Treg, Th17, and T follicular helper that can both promote and inhibit the behavior of each other. The lymphokine IL-15 is an important IL-2-related cytokine whose role in helper subset differentiation and proliferation has not been fully elucidated. In this study, we show that exogenous IL-15 decreased IL-17A production in Th17 differentiating cultures. Neutralization of IL-15 led to increases in IL-17A production in Th17 cultures. Both Il15(-/-) and Il15r(-/-) T cell cultures displayed higher frequency of IL-17A producers and higher amounts of IL-17A in the supernatants compared with those of wild-type (WT) cells in vitro. IL-15 down-modulated IL-17A production independently of retinoic acid-related orphan receptor-&#947;t, Foxp3, and IFN-&#947; expression. Both Th17 cells and APCs produced IL-15, which induced binding of STAT5, an apparent repressor to the Il17 locus in CD4 T cells. Also, in a model of myelin oligodendrocyte glycoprotein-induced experimental autoimmune encephalomyelitis (EAE), Il15(-/-) mice displayed exacerbated inflammation-correlating with increased IL-17A production by their CD4(+) T cells-compared with WT controls. Exogenous IL-15 administration and IL-17A neutralization reduced the severity of EAE in Il15(-/-) mice. Taken together, these data indicate that IL-15 has a negative regulatory role for IL-17A production and Th17-mediated inflammation.